3GPP Evolved UMTS Terrestrial Radio Access (E-UTRA) supports both FDD (full duplex) and TDD (half duplex) duplex modes. The behaviour of terminals simultaneously connected using different duplex modes has, however, not been specified. For network operators with both FDD and TDD spectrum, mechanisms that enable simultaneous use of both FDD and TDD spectrum resources, and thus allow both spectrum resources to be well (preferably fully) utilized, are desirable. As such, efficient TDD and FDD spectrum usage, and utilization of different technologies jointly, are expected to become more important for future LTE deployments in order to accommodate increased throughput and capacity needs.
The use of carrier aggregation (CA) offers a means for increasing peak data rates and throughput by aggregating multiple carrier components, as was discovered during 3GPP Release 10 LTE development, and enhanced during 3GPP Rel. 11 LTE CA enhancement work. It is expected that future LTE FDD-TDD CA deployment scenarios may allow either TDD or FDD cells to be used as a PCell and therefore, support for generic LTE FDD-TDD CA is desirable.